In manufacturing semiconductor devices, the semiconductor wafer is modified at various regions by diffusing or implanting positive or negative ions referred to as dopants, such as boron, phosphoric, arsenic, antimony and the like into the body of the semiconductor wafer to thereby produce regions having varying conductivity.
Various ion implanters are known, using several types of ion sources. An ion beam of a preselected chemical species is generated by means of a current applied to a filament within an ion source chamber, also fitted with a power supply, ion precursor gas feeds and controls. The ions are extracted through an aperture in the ion source chamber by means of a potential between the source chamber, which is positive, and extraction means. Allied acceleration systems, a magnetic analysis stage that separates the desired ion from unwanted ions on the basis of mass/charge and focuses the ion beam, and a post accelerated stage and insures delivery of the required ions at the required beam current level to the target or substrate wafer to be implanted, complete the system.
The most common type of ion source used commercially is known as a Freeman source. In the Freeman source, the filament, or cathode is a straight rod, typically made of tungsten or a tungsten alloy or other known materials, that is passed into an arc chamber, whose walls form the anode.
The arc chamber itself is fitted with an exit aperture, with means for feeding in the gaseous precursors for the desired ions; with vacuum means; with means for heating the cathode to about 2000 K up to about 2800 K so that it will emit electrons; with a magnet that applies a magnetic field parallel to the filament to increase probability of collision for ionized species; and with a power source connecting the filament to the arc chamber.
The ion implantation process begins with the evolution and extraction of ion species from a source chamber. Both solid sources (with vaporizers) and gaseous sources, such as BF.sub.3, AsH.sub.3, PH.sub.3, GeH.sub.4, SiC,.sub.4 and GeF.sub.4 are used. The source materials are passed over the filament, which is at elevated temperatures, and which decomposes the source material into the desired ionized species, that is, the compound that will be implanted, along with ions from the remainder of the source material compound. For example, AsH.sub.3 decomposes into As, H, and AsH.sub.x species. BF.sub.3 decomposes into B, BF.sub.2, F and other BF.sub.x compounds.
The problem caused by the F (fluorine) is that the F ions etch away tungsten from the source cavity, forming gaseous WF.sub.6. This material then diff-uses to, and decomposes on the surface of the filament. This results in metallic tungsten being deposited on the hot filament, fluorine ions being liberated. This metallic tungsten deposition on the filament causes the filament to increase in cross-sectional area, resulting in decreased filament resistance. The power input to the source must remain constant. If the filament resistance decreases, then the filament current must be increased to maintain the required constant power. Ultimately the implant power supplies cannot supply sufficient current to maintain this required power, and the source must be rebuilt with a new filament.
There is a related problem for non-fluorinated source materials, where tungsten is sputtered off of the filament, decreasing its cross sectional area. The filament will soon be too thin and will break, again resulting in the need to rebuild the source with a new filament. This sputtered-away tungsten also causes a problem in that it will deposit on the surface of the insulators that electrically isolate the various parts of the implant source. This will cause premature insulator failure and again result in the need to rebuild the ion source.
The time spent doing these source changes is a major cost-of-ownership driver for ion implanters. In some cases, such as if only GeF.sub.4 were run on a tool, the source must be replaced every 30 hours. Changing the source takes a significant amount of maintenance labor and can take up to 4-6 hours of tool down-time to complete.
One of the common methods used to extend source lifetimes is to alternate implant species. For example, a fluorinated source feed material, such as BF.sub.3 or GeF.sub.4, will be run for 4 hours, then a non-fluorinated species, such as AsH.sub.3 or PH.sub.3, will be run for 4 hours, then the tool will be switched back again to the fluorinated source material. The filament cross-sectional area will increase with the GeF.sub.4, then decrease with the AsH.sub.3, then increase again with fluorinated source material. The fluorine also helps to improve source life by scavenging sputtered away tungsten and causing it to be redeposited on the filament. This cycling results in increased source lifetime and improves overall implanter throughput. The problem with this is that it is logistically very complex to execute. There are also many instances where the daily production schedule does not support this optimal cycling sequence, making it impossible or impractical to run these alternating species. Furthermore, switching between source materials takes time and reduces the amount of time that the tool is available to run production.
It would therefore be desirable to provide a process for ion implanting dopants that extends the lifetime of the filament without a concomitant loss in throughput of the ion implanter apparatus.